Solar cell modules are devices that convert light energy from the sun into electrical energy, and are attracting attention as systems capable of generating electrical power without discharging carbon dioxide. These solar cell modules are required to not only have high electric power generation efficiency, but also have durability that enables them to withstand long-term use even in cases when used outdoors.
FIG. 3 is a schematic longitudinal cross-sectional view showing an example of a solar cell module.
This solar cell module 50 is mainly composed of solar cells 40 serving as photovoltaic power generation elements, a sealing material 30 serving as an electrical insulator that prevents short-circuiting of electrical circuits, a front protective sheet (front sheet) 10 laminated onto the front of the sealing material 30, and a back protective sheet (back sheet) 20 laminated onto the back of the sealing material 30.
In order to impart weather resistance and durability to the solar cell module 50 enabling it to withstand outdoor and indoor use over a long period of time, it is necessary to protect the solar cells 40 and the sealing material 30 from wind and rain, humidity, dust and mechanical impacts while also maintaining the inside of the solar cell module 50 in a sealed state that is isolated from the outside air. Consequently, the front sheet 10 and the back sheet 20 are required to have superior weather resistance, and are required to have a low level of water vapor permeability in particular.
In addition, it is also necessary for the back sheet to have durability and light reflectivity.
Back sheets have been previously proposed which, together with forming a fluorine resin layer on a base sheet, have an inorganic pigment such as titanium dioxide (TiO2) or silica (SiO2) dispersed in the fluorine resin layer in order to give the back sheet durability, weather resistance, light reflectivity and water vapor impermeability. For example, Patent Document 1 discloses a back sheet of a solar cell module employing a structure in which a cured coated film of a curable functional group-containing fluorine polymer coating material is formed on at least one side of a water-impermeable sheet, and a pigment such as titanium dioxide is uniformly dispersed in the cured coated film of the fluorine polymer coating material.